Born to a farming family in Mercy - le - Haut, Meurthe - et - Moselle, he attended the École Polytechnique and the École des Mines, graduating from both at the top of his class. He then became a mining engineer in Vesoul and Nancy, but left that profession at the age of 29 to enter politics. Lebrun gained a seat in the Chamber of Deputies in 1900 as a member of the Left Republican Party, later serving on the cabinet as Minister for the Colonies from 1912 – 1914, Minister of War in 1913 and Minister for Liberated Regions, 1917 - 1919. Joining the Democratic Alliance, he was elected to the French senate from Meurthe - et - Moselle in 1920, and served as Vice President of the Senate from 1925 through 1929. He was president of that body from 1931 - 1932.

Lebrun was elected president of France following the assassination of president Paul Doumer by Pavel Gurgulov on 6 May 1932. Re-elected in 1939, largely because of his record of accommodating all political sides, he exercised little power as president. On 10 July 1940, Lebrun was replaced by Philippe Pétain (although Lebrun never officially resigned) as head of state by a vote of the parliament.

He then fled to Vizille (Isère) on 15 July, but was captured on 27 August 1943 when the Germans moved into the region and was sent into captivity at the Itter Castle in Tyrol. On 10 October 1943 he was allowed to return to Vizille due to poor health, but was kept under constant surveillance.

On 9 August 1944, when the Allies restored the French government, Lebrun met with Charles de Gaulle and acknowledged the General's leadership, saying that he had not formally resigned as president because the dissolution of the National Assembly had left nobody to accept his resignation.

After the war, Lebrun lived in retirement. He died of pneumonia in Paris on 6 March 1950 after a protracted illness.